The present invention relates to a stud welding apparatus, and more particularly to a mechanism for mounting flanged studs, having a flange and a shank, to the tip of a welding gun.
In Laid-Open Utility Model Application [Jikkai] No. S62-202977/1987, gazette publication (Patent Literature 1), a stud welding apparatus is disclosed which has a mechanism for mounting a flanged stud, having a flange and a shank, to the tip of a welding gun. In that stud welding apparatus, the collet for mounting a flanged stud to the tip of the welding gun is configured by a tubular first collet member that is secured to the tip of the welding gun for elastically holding the flange to the fore, and a tubular second collet member that is secured to a piston which extends from an air cylinder at the aft end of the welding gun toward the tip of the welding gun for picking up, accommodating, and holding the shank of a stud that has been fed inside the first collet member. By forming the collet of two tubular collet members, namely a first and second collet member, even if the stud is flanged, the amount of deformation will be diminished when the flange of the first collet member that is of an externally tubular shape extends and emerges, fatigue and breakage can be prevented, and the holding force is increased and electric power supply performance is enhanced because the stud is held by the second collet member as well as by the first collet member. A stud welding apparatus wherein a collet is formed of a first collet member and a second collet member is also disclosed in Laid-Open Utility Model Application No. S63-196368/1988, gazette publication (Patent Literature 2) and Laid-Open Utility Model Application No. S62-155963/1987, gazette publication (Patent Literature 3).
The stud welding apparatus described in Patent Literature 1 exhibits advantages such as preventing collet breakage and fatigue when mounting flanged studs to a welding gun, as noted above. However, in order to pick up, accommodate, and hold the tip of a stud shank that is at the surface of the inner wall of the first collet member with the second collet member, it is necessary to form a tapered part in the tip of the stud shank. This has resulted in the stud becoming longer. A long stud is disadvantageous in that it makes the welding gun longer, and it has not been possible to handle cases where it is desired to use short studs. The points of needing a tapered part at the tip of the stud shank and of the stud becoming longer are described with reference to FIG. 1 and FIG. 2. In FIG. 1, a stud 2 delivered to a first collet member 1 of a welding gun is placed inside the first collet member 1 with the flange 3 to the fore, and the tip of the shank 4 is in such condition that it is in contact with the inner wall surface of the first collet member 1. In this condition, compressed air is supplied to an air cylinder 5, a piston 6 moves toward the first collet member 1 end, and a second collet member 7 at the tip of a piston rod 6A picks up the tip of the stud shank 4 and accommodates and holds the shank inside the second collet member 7. However, because the tip of the stud shank 4 is in contact with the inner wall surface of the first collet member 1, even if the tip of the second collet member 7 has a tapered part 8, as diagrammed, it is very difficult to pick up unless floating sufficiently free of the inner wall surface of the first collet member 1 due to the taper of the tip of the stud shank 4.
FIG. 2 is a detailed view of a stud 2 capable of use in a welding gun like that diagrammed in FIG. 1. Unless the overall length L of the stud 2 was made larger by a fixed multiple of the diameter D of the flange 3, the stud 2 could not be picked up even if the tapered part 9 was formed in the tip of the shank 4. This was so because, when the overall length L of the stud was short, the gap necessary for picking up the tapered part 8 in the second collet member could not be secured between the tapered part 9 at the tip of the shank 4 and the inner wall surface of the first collet member 1. It is known from experience that it becomes easy for the tapered part 8 of the second collet member 7 to pick up the shank 4 when the angle α1 of the tapered part 8 at the tip of the second collet member 7 is made 45 degrees or less and the outer diameter of the second collet member 7 is made as close as possible to the inner diameter of the first collet member 1. It is also known from experience that, in order to stably hold and weld a stud, the length L of the stud 2 should be L=F+b+L2+a≧1.4 D.
Here, as indicated in FIG. 3, F is the length necessary for the second collet member 7 to hold the shank, b is the length of the tapered part 8 of the second collet member 7, L2 is the length of the tapered part 9 at the tip of the shank of the stud, and a is the length of stud protrusion from the tip of the first collet member 1 to the edge surface of the flange 3 (length necessary for welding). For example, the stud holding length is F≧4 mm, the stud protrusion length is a=3 mm, and fixed lengths are required therein, respectively.
Accordingly, in order to shorten the stud length L, it is only necessary to shorten the length b of the tapered part 8 of the second collet member 7 and the length L2 of the tapered part 9 at the end of the stud shank. However, in the case of the welding gun of the configuration noted in Patent Literature 1, for the reasons given above, b and L2 cannot be made shorter than a certain length. In Patent Literature 2 and Patent Literature 3, in order to pick up the tip of a stud shank with the second collet member, a configuration is adopted for supplying compressed air, separately from the compressed air supplied in the first collet member to the stud, for maintaining the attitude of the stud horizontal inside the first collet member, and thus aiding the second collet member in picking up the stud shank. With such a structure, the length L2 of the tapered part 9 at the tip of the stud shank can be shortened, but the structure of the welding gun and the control of the supply of compressed air are made complex. In Patent Literature 2, furthermore, there is also a configuration wherewith a stopper is deployed midway in the stud supply path of the first collet member to stop the stud feed and maintain the stud attitude horizontal. However, there is a danger of blocking the stud feed midway along the feed path.